Sintered electric contact of high contact-fusing resistance



4, 1964 H. SCHREINER ETAL 3,143,626

SINTERED ELECTRI C CONTACT OF HIGH CONTACT-FUSING RESISTANCE Filed March 8, 1965 United States Patent ice 3,143,626 SINTERED ELECTRIC CONTACT OF HIGH CONTACT-FUSING RESISTANCE Horst Schreiner, Nurnberg, and Alfred Lehmann, Am-

berg, Germany, assignors to Siemens-Schuclrertwerke Aktiengesellschaft, Berlin-Siemensstadt, Germany, a corporation of Germany Filed Mar. 8, 1963, Ser. No. 263,815 Claims priority, application Germany Mar. 15, 1962 Claims. (Cl. 200-1645) Our invention relates to electric contact structures for opening and closing electric circuits in switching devices such as contactors, relays, selector switches, disconnect switcha, circuit breakers or the like.

Aside from silver and copper, having long since been used as well solderable electric contact metals, various other contact metals have become known more recently as superior in some respects, particularly as regards resistance to sticking, freezing or welding at the contact surface. In this respect, compositions of silver and lead have been found especially advantageous. But the solderability of this material is much inferior to that of silver and copper so that it is often difficult to obtain a satisfactory adhesion or attachment to a contact carrier. For coping with such difficulties use has been made of two-layer contacts produced for example by rolling-mill cladding methods and formed of a silver-lead composition in one layer and a well solderable metal in the other layer. These contact structures have the disadvantages that, when they are being hard-soldered to a carrier structure, the interposed solder metal and flux tend to reach and impair the contact layer.

It is an object of our invention to provide electric laminated contact structure which eliminates such trouble and loss in reliability while also atfording a particularly high resistance to sticking and welding at the contact surface.

According to our invention the contact body consists essentially of two jointly sintered and mutually sinterbonded layers of which one constitutes the contact layer and consists of a silver-lead composition having a porosity between 5 and 20 volume percent, whereas the other layer constitutes a solderable backing layer and is formed of pure silver, copper, or a silver alloy whose silver content by weight is preponderant. The solderable backing layer, being also porous in its interior, is densified at its rear surface by smearing, such as produced for example when subjecting the surface to pore-closing sanding or grinding. Preferably the thickness of the Ag-Pb layer is at least one-half the total thickness of the twin-layer structure and has a Pb-content of 2 to 20% by weight.

An embodiment of a contact structure according to the invention is schematically illustrated on the drawing by a sectional view.

The illustrated contact is circular and comprises a contact layer 1 consisting of the above-mentioned Ag-Pb composition, and a solderable layer 2 consisting entirely or predominantly of silver or of copper. Denoted by 3 is the boundary zone between the contact layer. The wavy shape schematically shown for the boundary zone 3 is intended to represent the serration and interlinking of the layer materials resulting from the fact that the two layers are jointly produced from respective powders by sintering which strengthens the coherence between the two layers, as will more fully appear from the following.

One way of producing a contact structure according to the invention is as follows. First, the powder or powder mixture for one of the two layers is filled into a matrix or die. Thereafter the predetermined space required for the powder of the other layer is provided by lowering the lower press punch in the die, whereby additional space in the die becomes available on top of the powder layer 3,143,626 Patented Aug. 4, 1964 first deposited. Then the second amount of powder is filled into the die on top of the first layer. The contents of the die is then jointly compressed. This produces a two-layer pressed body in which the two layers are well joined with each other by intimate inter-hooking. By subsequent sintering the pressed and shaped body is solidified essentially by volume diffusion. During sintering a portion of the lead diffuses from the Ag-Pb layer into the solderable layer.

For example, the starting mixture of the Ag-Pb composition may contain these two constituents in the ratio 95% to 5% by Weight. The second layer may consist of pure silver. Under these conditions, when producing a contact in which the Ag-Pb layer had an ultimate thickness of 0.9 mm. and the silver layer a thickness of 0.4- mm., the lead content in the contact layer after sintering was found to be 3.5 to 3.8% by weight instead of the original 5%. Under otherwise the same conditions but when using a starting mixture in the Ag/Pb ratio of 92.5/7.5% by weight, the lead content in the content mixture after sintering was 4.5 to 5%, and when using an Ag/Pb ratio of 10%, the lead content in the ultimate contact layer was about 6%. The quantity of lead diffused into the silver layer depends upon the layer thickness ratio d /d and decreases with an increasing value of this ratio. The data exemplified in the foregoing relate to a thickness ratio of 2.25.

The thickness of the soldering layer is preferably at least 0.1 mm. With smaller layer thicknesses there occur irregularities in layer thickness when using automatic filling devices in the manufacturing process. The upper limit for the thickness of the soldering layer depends upon the shape of the contact structure, particularly upon the total thickness of the structure and consequently upon the thickness of the contact layer. Particularly advantageous is a thickness of the soldering layer between 0.2 and 0.5 mm.

The contact body according to the invention is preferably mounted on a contact carrier by hard-soldering or brazing. For example when the copper carrier of 2.5 mm. thickness is used, it can be heated by high-frequency induction heating to the necessary brazing temperature. To avoid excessive heating of the contact layer, it is preferred to heat the carrier from the side remote from the contact layer. Silver hard-solder compositions with about 40% by weight of silver and about 20% each of copper, zinc and cadmium have been found particularly suitable. The commercially available fluxes for hardsoldering or brazing are applicable.

The porosity of the contact layer results in a relatively high fusion resistance at the contact surface. The closing of the pores by smearing of the solderable layer at the rear side prevents that liquid metal is sucked into the contact body during brazing. Due to the high plasticity and the cold-welding tendency of the above-mentioned materials the smearing of the surface zone can be etfected in a simple manner mechanically, for example by sanding.

The two-layer contact according to the invention can be produced in its ultimate shape of any desired geometric configuration, for example as a twin-layer rivet contact.

A production example for a contact according to the invention will be more fully described presently.

Electrolysis silver powder having a grain size below 37a is mixed with lead powder obtained by ejecting molten lead under pressure out of a pressure nozzle. The mixing ratio is to 5% by weight. In a matrix die having a pressure area of 8 x 7 mm. the lower pressure punch is adjusted to a filling height of 1.7 mm. The die space is filled with pure silver powder to form the soldering layer. Thereafter the lower punch is lowered a further distance of 4.7 mm., thus providing an additional filling space on top of the silver layer. This additional space is filled with the Ag-Pb powder mixture. Thereafter the total content of the die is pressed at 1.5 t./cm. (t. =metric ton). The pressed body has a density of 7.8 g./cm. which corresponds to an average space filling degree of 0.74 (the space filling degree indicates the ratio of press density and theoretical density which, for the 95/5 mixture of Ag-Pb amounts to 10.53 g./cm. and for pure silver amounts to 10.5 g./cm. The pressed and selfsupporting body is then sintered at 700 for thirty minutes in hydrogen. This results in linear shrinking of about 6%. Ultimately a sintering density of 8.9 g./crn. is reached, corresponding to a space filling degree of 0.844. Consequently, the median porosity of the contact body is 15.6%. The surface of the silver layer is then smeared 'by grinding on sand paper or Carboru'ndum paper. Thereafter the body, Without further treatment, is brazed onto a copper carrier of 1.5 mm. thickness. Used for this purpose is a hard-silver composition of 40% by weight silver and 20% by weight each of copper, zinc and cadmium, together with commercial flux. The heating of the carrier is effected with high-frequency induction. This results in satisfactory brazing without segregation of lead from the contact body.

The production method is performed analogously when forming the soldering layer of copper or a silver alloy having a preponderant silver content (more than 50% by weight). An example relating to such a silver alloy will be described presently.

Electrolysis silver powder having a grain size below 37a is mixed with lead powder obtained in the above-described manner, in the ratio of 90/ by weight. For preparing the soldering layer, electrolysis silver powder and electrolysis copper powder, each in a grain size below 37,41. are mixed in the ratio of 60/ 40% by weight. In a die of mm. diameter, the lower punch is first adjusted to 2 mm. filling height, and the available die space is then filled with the silver-copper powder. Then the lower punch is lowered 8 mm., down to a total filling. height of 10 mm. The adidtional die space of 8 mm. height is filled with the silver-lead mixture to produce the contact layer. The two layers are jointly densified at a pressure of l t./cm. to form a cylindrical pressed body. The density of this body is 7.0 g./cm. corresponding to a median space filling degree of 0.67. The body is then sintered at 650 C. for thirty minutes in hydrogen. The

occurring linear shrinking is approximately 5.5%. The density of the body after sintering is 8.32 g./cm. corresponding to a median space filling degree of 0.80. By after-pressing the density is increased to 9.88 g./cm. The further processing of the two-lamination body thus produced is as described in the preceding example. When hard-soldering or brazing the contact body to a carrier at a temperature of about 650 C., no lead segregates out of the contact body.

We claim:

1. A sintered electric contact body of high welding resistance, comprising two layers sinter-bonded to each other, one of said layers forming an electric contact surface and consisting of a silver-lead composition and having a porosity between 5 and 20 volume percent, said other layer forming a solderable backing and consisting of metal selected from the group consisting of silver, copper and silver alloys having a predominant silver content, said backing layer beingporous in its interior but dense at the surface remote from said one layer.

2. In a sintered laminated contact according to claim 1, said silver-lead layer containing 2 to 20% by Weight of lead.

3. In a sintered laminated contact according to claim 1, the thickness of said silver-lead layer being at least onehalf the total thickness of the contact body.

4. In a sintered laminated contact according to claim 3, said solderable backing layer having a thickness of 0.2 to 0.5 mm.

5. A twin-lamination sintered electric contact body, comprising a contact layer and a solderable backing layer sinter-bonded to each other, said contact layer forming a contact-fusion resistant contacting surface and consisting of a sintered silver-lead composition having a lead content of 2 to 20% by weight and a porosity of 5 to 20 percent by volume, said backing layer being formed of metal selected from the group consisting of silver, copper and silver alloys having a predominant silver content, and said backing layer being porous in its interior but substantially poreless at its solderable surface remote from said contacting surface, the thickness of said backing layer'being smaller than that of said contact layer and beingabout 0.2 to about 0.5 mm.

No references cited. 

1. A SINTERED ELECTRIC CONTACT BODY OF HIGH WELDING RESISTANCE, COMPRISING TWO LAYERS SINTER-BONDED TO EACH OTHER, ONE OF SAID LAYERS FORMING AN ELECTRIC CONTACT SURFACE AND CONSISTING OF A SILVER-LEAD COMPOSITION AND HAVING A POROSITY BETWEEN 5 AND 20 VOLUME PERCENT, SAID OTHER LAYER FORMING A SOLDERABLE BACKING AND CONSISTING OF METAL SELECTED FROM THE GROUP CONSISTING OF SILVER, COPPER AND SILVER ALLOYS HAVING A PREDOMINANT SILVER CONTENT, SAID BACKING LAYER BEING POROUS IN ITS INNER BUT DENSE AT THE SURFACE REMOTE FROM SAID ONE LAYER. 